For a pulsar with \(P=0.25\)  s and \(h_{\rm em} = 300\)  km, \(\rho \approx 14\) and for \(\alpha \)  not too low then \(W_{10} \approx 28\)  and \(\Delta _\phi \approx 6\) . Alternatively, measurements of \(\Delta \phi \)  and/or \(W_{10}\)  can help constrain \(h_{\rm em}\) . This approach, and its pitfalls, are outlined in e.g. [1]}. It should be noted that the standard picture of a hollow cone of emission [2]}, [3]}, [4]}, [5]} has come under close scrutiny, and an alternative model which invokes `flux tubes' or `fan-beams' is equally good at reproducing the observed phenomenology [6]}, [7]}, [8]}, [9]}. From a theoretical perspective, Equation REF  assumes a circular polar cap and no distortion of the magnetosphere, both of which are questionable (e.g. [10]}, [11]}, [12]}).
